It is known to be advantageous in certain circumstances to recover work from nitrogen produced in a cryogenic air separation plant. Most proposals for so doing are dependent upon the presence of a gas turbine employed to drive an alternator to generate electricity. See for example U.S. Pat. Nos. 2,520,862 and 3,371,495 in which compressed nitrogen is employed to control the pressure in the combustion chamber associated with the gas turbine, and energy is then recovered in the expansion of the gas. Accordingly, most if not all of the energy requirements of the air separation process can be met thereby. Frequently, however, a suitable gas turbine is not available on site to enable such processes to be used.
In UK patent specification 1,455,960 there is described an alternative process for recovering work from the nitrogen product. This method involves a thermodynamic linking of the air separation plant with a steam generator. The nitrogen product is heat exchanged with flue gases intended for generation of the steam in the steam generator so as to impart high grade heat to the nitrogen product and thus heat it to a temperature greater than 600.degree.. The nitrogen is then work expanded to convert most of its required heat energy into the mechanical energy. Steam is generated by the flue gases downstream of their heat exchange with the nitrogen product. Residual, available heat in the work-expanded nitrogen product is used to reheat fluids re-entering the steam generator.
The process described in UK patent specification 1,455,960 has a number of drawbacks. First, the use of high-grade heat to raise steam is relatively inefficient. Second, there is a significant cost involved in steam raising. Third, although there is the potential to use work recovered from the air separation process to generate large excess quantities of electricity for export, the process according to UK 1,455,960 does not avail itself to this possibility. Fourth, suitable steam generation plant may frequently not be available on the site of the air separation plant. Fifth, there may not be readily available a suitable source of high grade heat, and if there is, there may be more efficient ways of using it. Sixth, the process is unable to utilize low grade heat which is more commonly available from industrial processes (but which is generally wasted or used only inefficiently for power generation).